It is well known to apply as a memory a change in optical characteristics caused by reversible phase change of a substance, and a technique using this has come into practice as phase change optical disks such as DVD-RAM. Specifically, recording, reproducing and rewriting of signals will be available by rotating a disk medium comprising a substrate on which a recording thin film for generating reversible phase change is provided, and by irradiating the disk medium with a laser beam drawn to a sub-micron size. In the case of a phase change optical disk, overwriting by means of a single laser beam is carried out. That is, irradiation is performed by modulating the laser power between a high level and a low level depending on the information signal, so that an amorphous phase is generated at a region irradiated with a high power laser beam while a crystalline phase is generated at a region irradiated with a low power laser beam. As a result, a signal array comprising the amorphous portion and crystal portion alternately is recorded on the disk. Since the amorphous portion and the crystal portion are different in the light transmittance and reflectance, the change in the state can be read as a change in the amount of the light transmittance or reflectance by continuously irradiating a laser beam on this signal array, in which the laser beam is attenuated not to change the recording film.
Such a phase change optical disk has some characteristics such as:    (1) it enables the performance of overwriting, i.e., recording a new signal while erasing an old signal by using only one laser beam; and    (2) it can record and reproduce a signal by using a change in the reflectance, based on a principle similar to that of a ROM medium. These characteristics lead to several merits such as simplifying a system construction and providing devices for general purposes, so that such phase change optical disks are expected to be applied widely.
Recording materials used for recording layers of phase change optical disks generally include chalcogenide semiconductor thin films based on chalcogen elements such as Te, Se and S. A method used in the early 1970s is crosslinking a T network structure for stabilizing an amorphous state by adding materials such as Ge, Si, As and Sb to a main component of Te. However, these materials would cause a problem. That is, when the crystallization temperature is raised, the crystallization speed is lowered remarkably, and this would make rewriting difficult. Alternatively, when the crystallization speed is increased, the crystallization temperature is lowered sharply, and thus, the amorphous state will be unstable at a room temperature. A technique suggested for solving the problems in the latter half of the 1980s is the application of a stoichiometric compound composition. The thus developed compositions include Ge—Sb—Te based materials. In—Sb—Te based materials, and GeTe based materials. Among them, Ge—Sb—Te based materials have been studied most since the materials allow phase change at high speed, substantially no holes will be formed even after repeated phase changes, and substantially no phase separation or segregation will occur (N. Yamada et al, Jpn. J. Appl. Phys. 26, Suppl. 26-4, 61 (1987)). An example of material compositions other than such stoichiometric compositions is an Ag—In—Sb—Te based material. Though this material is reported to be excellent in the erasing performance, it has been found that the characteristics deteriorate due to the phase separation as a result of repeated overwriting.
Similarly, characteristic deterioration caused by repetition may be observed even if a stoichiometric composition is used. An example of the deterioration mechanism is a phenomenon of micro-scaled mass transfer caused by repetition of overwriting. More specifically, overwriting causes a phenomenon that substances composing a recording film flow little by little in a certain direction. As a result, the film thickness will be uneven at some parts after a big repetition. Techniques to suppress the phenomenon include the addition of additives to recording layers. An example of such techniques is addition of a N2 gas at a time of film formation (JP-A-4-10979). A document clarifies a mechanism that a nitride having a high melting point is deposited like a network in a grain boundary composing the recording film, and this suppresses the flow (R. Kojima et al. Jpn. J. Appl. Phys. 37 Pt. 1, No. 4B. 2098 (1998)).
JP-A-8-127176 suggests a method of including a material having a melting point higher than that of the recording material.
As mentioned later, the cited reference is distinguishable from the present invention in that the material having a high melting point will not be dissolved in the base material but scattered in the base material layer. According to the reference, the scattered material having a high melting point suppresses the mass transfer phenomenon caused by repeated overwriting so as to improve the performance. JP-A-7-214913 suggests, without clarifying the mechanism, the addition of small amounts of Pt, Au, Cu, and Ni in a Ge—Sb—Te film in order to improve stability of the amorphous phase without lowering the repeatability.
However, the repetition number tends to decrease when the recording density is increased. Due to a recent demand for keeping compatibility among media of various generations, recording at higher density should be performed by using optical heads of identical performance (i.e., laser beams of an identical wavelength and object lenses of an identical numerical aperture). The size of a recording mark should be reduced to raise recording density. On the other hand, the strength of reproduced signals is lowered as the recording mark becomes small, and the signals will be influenced easily by a noise. Namely, during a repeated recording, even a slight variation that may have not caused a trouble in a conventional process will lead to errors in reading, and thus, the number of available repetitions of rewriting is decreased substantially. This problem can be noticeable in the a case of so-called land-groove recording, in which a concave-convex-shaped groove track is formed on a substrate and information is recorded on both the groove (a region closer to the light-incident side) and the land portion (spacing between the grooves) in order to guide a laser beam for recording and reproducing. Specifically, since the thermal and optical conditions are different between the land and groove, the repeatability will deteriorate easily, especially in the land region.
Merits provided by a recording layer comprising a compound material have been described above. On the other hand, when the composition of the recording layer is changed from the stoichiometric composition, the recording performance will be changed remarkably. In a desirable recording method, the performance of a recording film should be controlled with further accuracy while keeping the merit, of the compound composition, and using an identical recording film or a composition having a wide acceptability with respect to characteristics.
Electrical switching devices comprising a chalcogenide material and memory devices are known as well as applications of such phase change materials. The electrical phenomenon was first reported in 1968. Specifically, when voltage is applied gradually to a phase change material thin film in an as-depo.-state sandwiched between electrodes, electrical resistance a between the electrodes sharply declines at a certain threshold voltage, and a large current will start to flow (crystallization). For reversing this state to an initial low-resistant state (OFF state), a big and short current pulse will be passed. A portion provided with current is melted first and then, quenched to be amorphous so that the electrical resistance is increased. Since differences in the electrical resistance can be detected easily by an ordinary electrical means, the material can be used as a rewritable memory. Though material compositions based on Te have been used for electrical memories, any of them require a μs order period of time for crystallization.